- After a flood, the channel along the north-eastern bank is widened and straightened into a well-developed funnel shaped tidal inlet. This situation existed in 1963, during the ECRU survey in November 1981 and also probably throughout most of 1981 after the floods earlier that year. During such conditioons the tidal inlet is close to the north-eastern bank, causing heavy erosion just below the railway line.
- Narrow channel meandering from the deep water area at the south-western side of the lagoon towards the north-eastern bank and to sea via a sand delta at the foreshore. The south-western sandpit is fully developed, pushing the channel towards the north-eastern bank (without causing erosion). The lagoon is apparently not tidal and the channel is merely an overflow of lagoon water. These conditions existed in 1957 and also during a visit to the Hartenbos by ECRU personnel at the beginning of December 1981.
- Mouth closed with a large sandbank on the north-eastern side and deep water on the south-western side. There is a solid beach bar across the entire mouth area. This closed-mouth condition existed when aerial and terrestrial photographs were taken in January 1940, December 1968, June 1969 amd April and October 1977. It has been the typical situation in the Hartenbos since construction of the Hartebeeskuil Dam in 1970. (Ref 1)
At times when the water level in the estuary has risen without breaking through at the mouth (usually at the end of winter), the sandbar at the mouth has been artificially breached. Motivation for artificial breaching usually emanates from people wanting to gain access to "Die Bult" holiday facilities situated on the floodplain on the northern bank of the estuary, just downstream of the railway bridge. The access path passes under the northern side of the railway bridge and it is here that the path first becomes inundated. After being breached the mouth usually remains open for about a week before it is closed by marine sediments again (GJ Lamprecht, GP Kellerman and GL de Lange, pers.comm.). (Ref 1)
In a review of the data on coastal currents in South African waters, Harris (1978) furthermore makes note of the relatively high percentage frequencies of both slack water and onshore currents, off the coast respectively west and east of Mossel Bay. He also highlights the fact that oil from the Venpet/Venoil collision in December 1977, moved inshore and was deposited along the coastline east of Mossel Bay. For four days prior to the deposition of the oil, the wind had a strong onshore component. (Ref 1)
The wave rose shows that the promontory of Cape St Blaize largely shelters the half-heart-shaped Mossel Bay from the prevailing swells. Hence the mouth of the Hartenbos River lies in an area which theoretically can receive only direct deep sea waves originating from the south south-east to the east. Waves approaching from a direction to the west of south south-east do, however, also enter Mossel Bay because they are diffracted in an anit-clockwise direction around Cape St Blaize.
All waves from the south-east, east south-east and east, approach the coast at Hartenbos undisturbed in an easterly direction. All waves from the south south-east, south, south south-west and south-west, after being diffracted around Cape St Blaize to a lesser or greater degree, also run up on the Hartenbos coastline in an easterly direction.
Because the diffracted waves that reach the Hartenbos coastline are low-energy waves and because there is a low percentage of deep-sea waves from the south-east that reach the Hartenbos coastline undisturbed, wave-generated east-bound longshore drift is minor or non-existent. In a diffraction area, the longshore currents are normally more complex and contain a component due to longshore variation in wave height within the diffraction area, which in turn gives rise to a longshore variation in wave set-up (wave set-up is the build-up of a higher mean water level inside the surf zone than outside, which is associated with the momentum flux in progressive waves). In the instance of Mossel Bay, this means that the water level in the surf-zone is higher at the mouth of the Great Brak than it is at Hartenbos. A gradient from east to west is created which results in a westward flowing surf-zone current and accordingly ina net longshore sediment displacement towards the west. Although it is possible that the net longshore drift at the mouth of the Great Brak River may change in direction, the longshore drift to the west of it and at Hartenbos is predominantly towards the west. This is because of the weakening of the east-bound component. (Ref 1)
The sediment movement and basic dynamics at the Hartenbos River mouth as could be established by the study of historical photographs, aerial photographs, the geomorphology, inshore oceanography, and relevant local information and the observations carried out on 12 November and 5 December 1981, led to the following conclusions:
- Before 1970 the estuary mouth underwent characteristic changes.
- The typical situation since the construction of the Hartebeeskuil Dam in 1970 was the solid beach bar across the entire mouth area.
- The mouth of the Hartenbos Estuary opens naturally only during major floods.
- During closure of the mouth the south-western sandspit grows from the south-west to the north-east with the result that the river course is diverted to the north-eastern bank causing severe erosion of the bank.
- In the light of the observations, any proposed developments such as the construction of a roadbridge across the estuary near the mouth and a road running below and parallel to the railway line must be discouraged. (Ref 1)